Machinery Lubrication

Machinery Lubrication July-August 2019

Machinery Lubrication magazine published by Noria Corporation

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Page 24 of 70

HYDRAULICS line size. Sometimes when a 2-inch hydraulic line is teed off to a 1/4-inch line, a draftsman may represent this as a fi xed orifi ce. If you remove the valves from a manifold, you will fi nd large holes and small holes drilled through it. e fi xed-orifi ce fl ow control symbol may represent one of the smaller holes. It may also represent an orifi ce that can be removed from the manifold with an Allen wrench. Whatever form it takes, it should never be removed and replaced by a variable-orifi ce fl ow control. e designer had something specifi c in mind when calling for a fi xed-orifi ce fl ow control. For whatever reason, the designer did not want it to be adjusted. Its purpose may be to synchro- nize fl ow to more than one actuator, or it may be for safety purposes to keep an actuator from moving too rapidly. Variable Orifi ce e variable-orifi ce fl ow control symbol in Figure 3 has a diagonal arrow to indicate that it can be adjusted. It often is called a needle valve because a common construction uses a conical needle that seats to close off the valve. e conical needle is called a vernier. e purpose of the vernier is to make the adjustment proportional to the number of turns made on the adjustment. A common number of turns between fully open and fully closed is fi ve, so each full turn of the knob will change the orifi ce size by 20 percent. Manual valves such as ball valves, gate valves and butterfl y valves should never be used to control speed in a hydraulic system. ese types of valves are meant to be open or closed. Keeping them partially open in a high-pressure hydraulic system will cause them to be unable to seat properly, and they will not close all the way. e vernier is specifi cally designed to adjust fl ow. ere are also cartridge-type fl ow controls that mount in a manifold or valve stack. While these are not true needle valves, they are engi- neered with a spool that is cut to limit fl ow. It is not uncommon for either fi xed- or variable-orifi ce fl ow controls to have a built-in bypass check valve (Figure 4). e fl ow control with a bypass will limit fl ow in one direction but will allow free fl ow in the opposite direc- tion. e purpose of these controls is usually to enable independent forward and reverse speed control. Pressure Compensating e pressure-compensating fl ow control (Figure 5) is designed to maintain a constant fl ow regardless of the pressure drop across it. is control is used in systems where the load weight changes, but it is important to maintain a constant speed. Without the pres- sure- compensating feature, a heavier load will move more slowly than a lighter load. e pressure-compensating fl ow control is available as either a fi xed- or variable-orifi ce type. Temperature Compensating If the ambient temperature varies enough to aff ect production, the answer may be a temperature-compensating fl ow control. e additional symbol looks a bit like a thermom- eter. is control can be expensive, so it is unlikely to be found unless it is truly needed. e temperature-compensating fl ow control will maintain a constant fl ow regardless of any changes in oil viscosity. It is available as either a fi xed- or variable-orifi ce type, and can also be a pressure-compensating control if the load weight changes constantly. When I am called to help diagnose prob- lems in a system and the issue concerns a fl ow control, the most common problem I fi nd is that an incorrect type of fl ow control has been installed. It may be that the fl ow control specifi ed by the designer failed to take certain conditions into account, or that a fl ow control has been replaced with one of the wrong types. If speed control is important to your operation, switching to one of these fl ow controls may help. ML About the Author Jack Weeks is a hydraulic instructor and consultant for GPM Hydraulic Consulting. Since 1997 he has trained thousands of electricians and mechanics in hydraulic trou- bleshooting methods. Jack has also taught radio-wave propagation for the U.S. Air Force and telecommunications equipment operation and repair for the Central Intelligence Agency at American embassies overseas. 20 | July - August 2019 | www . Figure 2. A fi xed-orifi ce fl ow control symbol Figure 3. A variable-orifi ce fl ow control symbol Figure 4. A fl ow control symbol with a bypass check valve Figure 5. A pressure-compensating fl ow control symbol

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